Turbine oils, especially aviation jet engine oils, are complex liquid lubricants designed to function under extreme conditions of temperature, pressure and load. At such conditions the turbine oil experiences volatilization of vital components as well as a loss of the ability to control or inhibit wear of critical engine parts.
To control wear, the liquid lubricant typically employs additives specifically designed as antiwear additives. Typical turbine antiwear additives are addressed and described in the prior art.
EP 0,521,628 is directed to a combination of aryl phosphates useful as antiwear additives in fuels, lubricants and functional fluids which are subjected to high temperatures under actual service conditions. The combination of aryl phosphates is comprised of: (1) at least one oil-soluble aryl phosphate of the formula:(RO)3POwherein each R is, independently, phenyl or an alkyl-substituted phenyl group; and (2) at least one oil-soluble aryl phosphate of the formula:
wherein each R is, independently, phenyl or an alkyl-substituted phenyl group, Ar is m-phenylene or an alkyl-substituted m-phenylene group, and n is a number from 1 to 4, said combination containing from 2 to 30% by weight of component (1). When the above formula represents a mixture of the depicted polyphosphates, n is a whole or fractional number from 1 to 4, because n in that case represents the average composition of the mixture.
The lubricant compositions which can be benefited by the use of the composition combination are lubricating oils of lubricating viscosity, including oils composed by volume of at least 50% up to and including 100% of one or more mineral oils or one or more synthetic ester oils.
In forming the combination of aryl phosphates, use can be made of phenol and/or one or more alkyl phenols containing from 1 to 5 alkyl groups on the ring. Each alkyl group can contain up to about 18 carbon atoms provided the alkyl substituents do not sterically hinder the hydroxyl group to such an extent that the substituted phenol cannot undergo reaction to form the desired aryl phosphate.
The combination of aryl phosphates is reported to have excellent thermal stability at high temperatures, even as high as 400° C., compared to tri-n-octyl phosphate (TOP), tricresyl phosphate (TCP), tri-n-butoxy-ethyl phosphate (TBEP), and cresyl diphenyl phosphate (CDP).
When used as lubricating oil antiwear additive, the combination is employed in the range 0.005 to 20 wt % based on the total weight of the finished lubricant. The finished lubricant can contain additional additives, including ashless dispersants, zinc hydrocarbyl dithiophosphates, one or more oil-soluble sulfur-containing antiwear and/or extreme pressure agents.
U.S. Pat. No. 5,560,849 is directed to improving the antiwear properties of synthetic ester lubricants intended for use at high temperatures (150 to 350° C.) by adding to the lubricant a hydrocarbon-insoluble, synthetic ester-soluble, aryl diphosphate ester composition containing a major amount of an aryl diphosphate ester of the formula:
wherein Ar is an unsubstituted or alkyl (C1 to C12)-substituted arylene and R is unsubstituted or alkyl (C1 to C12)-substituted aryl.
The alkyl diphosphate ester exhibits high thermal stability, making it suitable for use as an antiwear additive in lubricants used at high temperature as a replacement for a tri-aryl phosphate ester (e.g., TCP).
The aryl diphosphate ester is employed in synthetic ester base stocks including polyol esters, diesters and phosphate esters in amounts in the range 0.1 to 10%, preferably 0.5 to 5%, based on the weight of the synthetic ester. Other additives may also be present in the synthetic ester lubricant and include lubricating agents, other antiwear additives, antioxidants, metal passivators, rust and/or corrosion inhibitors, viscosity index (VI) improvers, detergents/dispersants, defoamers/antifoamants, emulsion modifiers, seal swell agents, tackifiers, stabilizers, dyes and odor-masking agents. Antioxidants include hindered phenols, aromatic amines and zinc dialkyl/diaryl phosphate (ZDDP).
EP 0,612,837 is directed to a polyphenylene ether lubricant containing hydrocarbyl bis(dihydrocarbyl phosphate) compounds of the formula:
wherein R is either unsubstituted or substituted hydrocarbyl alkyl or aryl group, A is a bridging group alkylene, haloalkylene, arylene, two arylene groups joined by a bridging group (e.g., —C(CH3), —SO2 or —CO) or a bridging group containing biarylene, the di-phosphate compound being used as an antiwear additive in an amount in the range of about 0.1 to 10% by weight of the lubricating composition.
U.S. Published Application 2007/0179069 is directed to high temperature, stable lubricant compositions comprising at least one polyol polyester derived from the reaction product of a neopentyl polyol with 5,7,7-trimethyl-2-(1,3,3-trimethylbutyl)-octanoic acid. This polyolester can be used in combination with other conventional polyolesters as well as in combination with other additives, including metal protecting additives such as t-butyl phenyl phosphate, mono-hexyl and di-hexyl phosphates, isopropylphenyl phosphates, tri-cresyl phosphates, tri-xylyl phosphates, di(n-octyl) phosphate, alkylated tri-phenyl phosphorothionate, tri-phenyl thiophosphate, benzotriazole, tolyltriazole and mixtures, derivatives and combinations thereof in amounts in the range of about 0.1 to 10%, preferably up to about 5%, by weight of the total composition.
Other lubricant additives include alkylated di-phenyl amines, nonylated di-phenylamine, styrenated di-phenyl amine and hindered alkyl phenols.
U.S. Pat. No. 5,206,404 is directed to a composition of alkylated phenyl phosphate esters comprising 1 to 20 wt % tri-alkyl phenyl phosphate, 10 to 50 wt % di-alkyl phenyl mono-phenyl phosphate, 15 to 60 wt % mono-alkyl phenyl di-phenyl phosphate and less than 2 wt % tri-phenyl phosphate exhibiting a combination of excellent thermal, oxidative and hydrolytic stability. Unexpectedly superior thermal and hydrolytic stability properties are obtained when the alkyl moiety is a tertiary alkyl, preferably a t-butyl moiety.
The phosphate esters are an ashless antiwear base stock of reduced volatility and find further use as a lubricant additive.
“Phosphate Reactions as Mechanisms of High Temperature Lubrication”, Nagarajan, A. et al., NASA/TM-2006-213060, is directed to the study of the chemical vapor deposition of tert-butyl phenyl phosphate (TBPP) on cast iron. TBPP is reported as possessing superior oxidative, thermal and hydrolytic stability compared to other commercial phosphate ester products and can be an effective and highly stable antiwear additive in both petroleum and synthetic base stocks including polyesters, diesters and polyalphaolefins.
See also “Chemical Vapor Deposition Applications in Thin Film Coatings for High Temperature Lubrication”, Nagarajan, A. et al., AIChE Annual Meeting, Conference Proceedings, Cincinnati, Ohio, Oct. 30-Nov. 4, 2005 (564a/1-564a/11); and
“Vapor-Phase Lubrication: Reaction of Phosphate Ester Vapors with Iron and Steel”, Johnson, D. W. et al., Chem Mater, 2002, 14, 3767-3775.
While tri-alkyl phenyl phosphates are useful as antiwear additives in polyester base turbine oil formulations containing monomeric amine antioxidants, their performance has been found to be no more beneficial than tri-cresyl phosphate when employed in turbine oil formulation containing polymeric amine antioxidants. In fact, their performance as antiwear additives is negatively impacted when employed in high performance turbine oil formulations containing polymeric amine antioxidants.
It would be an advance in turbine oil technology if a way could be found to retain high antiwear performance in turbine oil formulations containing polymeric amine antioxidants. It would also be an advance in turbine oil technology if a way could be found to improve the deposition formation resistance performance of turbine oils containing monomeric or polymeric amine antioxidants.